Effective disinfecting processes are necessary for the treatment of bulky amounts of polluted materials such as water, especially domestic and industrial circulating waters, and aqueous effluents (such as being present in the foodstuff processing industry) containing micro-organisms which cannot be discharged or re-used untreated for hygienic, operational or environmental reasons. Effective disinfecting processes are also necessary for treating surfaces such as premises, equipment, containers, air-conditioning systems and the like. Environmentally compatible disinfecting processes are mainly based on the use of active oxygen compounds, such as hydrogen peroxide, or monomeric quaternary ammonium compounds.
Hydrogen peroxide is a moderately active, mild disinfectant with bactericidal properties. Hydrogen peroxide concentrations of 25 mg/l are known to inhibit the growth of some bacteria, however an effective reduction of the germ count, even at a much higher hydrogen peroxide concentration, takes many hours or requires additional ultraviolet radiation. Generation of the latter, however, requires both expensive equipment and substantial electricity costs. Therefore when disinfecting large amounts of polluted materials such as water, for instance for the treatment of water in sewage works and their outputs, such measures are practically inadequate and/or uneconomic. Therefore, various ways to overcome these disadvantages have already been tried in the art.
It is well known in the art that silver ions and silver-based compounds are highly toxic to micro-organisms, therefore showing strong bactericidal effects in many common species of bacteria including Escherichia coli. It has also been showed that hybrids of silver nanoparticles with amphiphilic hyperbranched macromolecules exhibit effective antimicrobial surface coatings. Stable aqueous dispersions of silver nanoparticles in the form of non-toxic elementary silver hydrosols were found to be strongly bactericidal for E. coli., a concentration of 50 μg/cm3 causing 100% inhibition of bacterial growth. Silver nanoparticles were found to accumulate in the bacterial membranes, somehow interacting with certain building elements of the bacterial membrane, thus causing structural changes, degradation and finally, cell death. The surface of bacteria is overall reported to be negatively charged, at biological pH values, due to the dissociation of an excess number of carboxylic and other groups in the membrane. It has been suggested that silver nanoparticles embedded in the membrane carbon-matrix generate a surface charge due to their movement and friction inside the matrix and in this way electrostatic forces might be a cause for the interaction of the nanoparticles with the bacteria. Furthermore, silver will tend to have a higher affinity to react with phosphorous and sulphur compounds contained in the bacterial membrane but also in DNA. A third possible mode of action is the release of silver ions which may further contribute to the bactericidal effect of silver nanoparticles. Several species of micro-organisms, e.g. Lactobacillus sp. and the fungus Fusarium oxysporum, have been reported to biosorb Ag(I) to their cell surface and detoxify this ion by reduction to Ag(0), either by reductase action or by electron shuttle quinones or both.
A non cytotoxic antimicrobial formulation comprising biologically stabilized silver nanoparticles in the size range of 1 to 100 nm, and a carrier in which the concentration of the said biologically stabilized silver nanoparticles is in the range of 1 to 6 ppm is already known in the art.
It is also known a method for preparing a colloidal silver-biomolecule complex comprising:                providing a mixture of a biomolecule, a silver salt, and a source of halide ions in a single solution; and        irradiating the mixture with light having a wavelength in the visible region, wherein the silver salt and source of halide ions are water soluble; the amounts of the bio-molecule, the silver salt and the source of halide ions being such that, the irradiating step results in formation of colloidal silver-bio-molecule complexes.        
It has also been disclosed a process for the preparation of nano-sized colloidal metal particles, said process comprising treating wet fungus or fungus extract with a metal ion solution at a temperature in the range of 15 to 40° C. for a time period ranging between 2 to 120 hours, and separating the biomass to obtain the nano-sized colloidal metal particles.
Conventional production methods for making silver nanoparticles have a number of disadvantages such as high production costs, the production of a significant proportion of by-products, or the existence of an upper limit for the concentration of nanoparticles obtained. For instance the latter production method requires a significantly high production time and is based on using fungus that may be pathogenic. Therefore there is a need in the art for a method for making silver nanoparticles that is reliable, inexpensive and reduces or avoids the formation of by-products.
An Ag(I) biosorption process by means of Lactobacillus, its pH dependency in the pH range from 2 to 6 and temperature dependency in the range from 10 to 60° C., as well as the mechanism of the reduction of Ag+ to Ag by Lactobacillus, has also been studied.
It is also known in the art a process for preparing silver nanoparticles by bioreduction using Aeromonas sp. in admixture with silver ions, ammonia and sodium hydroxide, at 60° C. during a couple of hours.
The above mentioned processes suffer from disadvantages like the elevated temperature, acidic pH and/or high incubation time required, or the insufficient bactericidal activity of the silver nano-particles resulting therefrom.
There is therefore a need in the art for producing silver or gold nanoparticles by a method which is free from these disadvantages.
There is also a need in the art for a simple, environmentally-friendly and reproducible method for producing silver or gold nanoparticles with high anti-microbial properties.
There is also a need in the art for a corresponding method for producing gold or silver nanoparticles which are known to be useful in certain medical applications.
Colloidal forms of metals other than gold or silver, and compounds of said metals, are also known in the art to have valuable properties and applications. For instance, colloidal bismuth subcitrate is water-soluble especially at a pH range from about 3 to 8 and has been used for decades for the treatment of gastric and duodenal ulcers, and Helicobacter pylori infection together with antibiotics. Colloidal forms of mercury, inorganic mercury compounds and metallic mercury ointments have been used topically for a variety of therapeutic uses including the treatment of infected eczema or impetigo (mercury salts), the treatment of syphilis (calomel), the treatment of psoriasis (mercuric oxide or ammoniated mercury). Colloidal forms of palladium and platinum have been used as catalysts for a variety of chemical reactions including organic reductions, hydrogenolysis and the like. Platinum nanoparticles in colloidal form are also known as anti-cancer agents. Colloidal copper, optionally chelated with salicylic acid, is a strong anti-inflammatory agent, and sublingual forms of colloidal copper or colloidal zinc are known as being active for fighting colds and flu. Also, colloidal zinc can be especially effective against viruses. In all these various fields there is a permanent need for providing alternative physical forms of the colloidal metals or colloidal metal compounds in order to improve their efficiency in their relevant fields of application.